I would expect bundles of DLC, but only after they are released. So, rather than a "buy 3, get 1 free" coupon, and wait for the next couple of DLC to be released, they'll instead offer something like: 25% off 4 existing DLC.
Pre-sales of DLCs are pretty much unheard of, for any game, because it's so unpredictable about when they'll come out, the quality of one, etc. So discounts tend to be applied to sales of EXISTING DLC packs, rather than future ones.